Electromagnetic radiation signals can be undesirably affected by signal drift. Generally, signal drift, also known as frequency drift, is an unintended offset of a signal's carrier frequency from its nominal frequency. Because of the inverse relationship between frequency and wavelength, frequency offset can be described using a wavelength of the signal. There are several causes for signal drift. For example, changes in temperature that can affect components of a system or overall breakdown of components leading to system failure can cause signal drift.
Signal drift can be particularly problematic if there are multiple signals sent at different wavelengths along the same connection. For example, optical signals having different carrier frequencies may be communicated along the same optical fiber. For example, a wavelength-division multiplexed signal uses different and discrete wavelength channels to communicate multiple sets of data along an optical fiber. When a signal drifts in a wavelength-division multiplexed signal, a signal drift of one channel, or wavelength, can cause that channel to drift into its adjacent channel causing interference. For optical signals, interference is caused by leaking photons from the drifting channel to another channel. Such interference can cause significant noise in a channel. Noise causes error rates of a signal or channel. Generally, it is desirable to minimize error rates of a signal or channel. A prolonged interference or substantial amount of interference can lead to unacceptable rise in the error rate. Increased error rate can lead to the total loss of data on the channel.
Wavelength-division multiplexing is a technique that multiplies signals so that multiple signals are carried together along a same fiber, each of the signals separated at different wavelengths or channels on the multiplexed signal. Wavelength-division multiplexing may be used to construct various types of networks. A ring topology network and a star topology network are just two of several known network topologies which can be created to utilize wavelength-division multiplexing to generate a multiplexed signal. A star topology network has a central hub and various nodes connected to the central hub with multiple inputs and outputs. Each node may generate a signal that is sent to the central hub, where the multiple signals are wavelength-division multiplexed and the multiplexed signals are sent back to the individual nodes. Accordingly, each of the nodes may detect the multiplexed signal, which generally allows each individual node to see the signals from all of the other nodes via the channels of the multiplexed signal. A passive star coupler is a star coupler that does not require any additional power to wavelength-division multiplex signals to generate a multiplexed signal. A ring topology may wavelength-division multiplex signals of multiple nodes in a serial fashion, wherein signals from each node are wavelength-division multiplexed at the leg of the ring topology from node to node, that can ultimately lead to a complete multiplexed signal at a leg of the ring topology.
For a system that uses multiple signals using multiple wavelengths along the same optical fiber, it would be advantageous to be able to detect signal drift. One possible method is to use a spectrum analyzer. A spectrum analyzer is used to examine the spectral composition of the optical waveform. For example, a spectrum analyzer can calculate a Fourier transform of a signal, resulting in a waveform in a power spectrum, wherein different frequency components of the waveform are shown as separate bands or channels over a given frequency range. Power or magnitude of each frequency component may also be shown in the power spectrum. However, there are several disadvantages of using a spectrum analyzer. Spectrum analyzers require expensive equipment and may not be able to be integrated into some systems or solutions. Further, because of signal-to-noise ratios, spectrum analyzers use long acquisition times and signal processing methods such as signal averaging. Further, the Nyquist frequency limit also may hinder perfect reconstruction of the signals from the waveform. To resolve this issue, further steps may be required, such as using different filters and oversampling. Further, use of these techniques may cause ghost signals, wherein certain frequency components are included in the output but were not part of the original signal.
Accordingly, improved methods for detecting signal drift are desirable.